The Citadel is the headquarters of Combine operations, a hug tower that is big onto the sun. It extends kilometres into the air and has a secure base some distance underground. It is assumed to be heavily armoured and impervious to external assault. The Citadel features a Dark Energy Reactor at the apex that enables the creation and sustenance of trans-dimensional portals. It is powered by a Dark Energy core underground that poses the danger of a catastrophic meltdown if not deactivated properly.
In DRAW Your Own Story 10, the Combine provided the Avatar Supreme Commander with a new command headquarters by planting a smaller-scale Citadel on top of the Slottet, which somehow was relocated to Moscow. CivGeneral, Alyx Vance, Thorvald of Lym and Bjørn stormed the Citadel at the end of the Avatar Wars. Dostya entered late, and nearly got herself killed before shooting the control computer and overloading the portal generator. The ensuing power surge destroyed the structure, which also overloaded the CivFanatics firewall. Fortunately, Thunderfall was restored to full ability soon enough to prevent a mass influx of ungainly folk, but the Combine used their existing toe-hold to teleport in reinforcements before their link to the Citadel's portal was severed.
The Moscow Citadel was too small to support the infrastructure needed for a Dark Energy core, and so its destruction did not pose a threat to the surrounding city. After the Eastern Union military was able to secure the Moscow Oblast in the aftermath of repeated attacks, the Citadel ruins were excised from the Slottet, which was returned to Oslo.
New Port City Citadel
During the Combine Occupation of Coruscant, the Combine plopped a citadel right on top of the old senate building in New Port City. The Citadel was used as Dr. Breen's base of operations until he was forced out by the liberating forces of the Republic of Coruscant.
The New Port City Citadel now serves as Coruscant's own defensive structure, marketed as the world's largest flak tower and base of operations for the Coruscanti Department of Defense and Department of Energy. Breen's old office has been converted into a long-range observatory post. The former location of the Dark Energy reactor at the top of the structure has been converted into a series of radio relay antennas and satellite dishes.
The Citadel lacks the ability to create interdimensional portals, as the Dark Energy reactor was relocated underground from its vulnerable perch on the roof. With the reactor converted for electrical power generation, the Dark Energy core provides the Republic of Coruscant with all the energy it needs. The power facility is subject to rigid safety regulations even tighter than for a traditional nuclear power plant to prevent a devastating collapse of the core.
Sometime after the events of the Third Nazi War, the New Port City Citadel received a great deal of war prizes consisting of biological agents including the Forced Evolutionary Virus, the T-Virus, and a Tyrant T-002 specimen, slated for "immediate destruction". After the Battle of Nar Kreeta, Dr. Edward Richtofen infiltrated the Citadel to acquire these spoils and let loose the Tyrant.
- Equivalent to the Hoover Dam wonder in the Civilization series